Two common reformer technologies are steam methane reforming and autothermal reforming. Both expose hydrocarbon gas, such as natural gas, to a supported catalyst at high temperature and pressure to extract the hydrogen. However, the autothermal reformer burns a portion of the hydrocarbon gas within the reforming vessel to provide heat for the reaction, whereas the steam methane reformer uses hot gases to externally heat tubes containing a mixture of steam and methane. The catalytic reaction in the autothermal reformer takes place in a single, large vessel. By contrast, the steam reformer chamber consists of more than a hundred parallel metal tubes running end to end inside a large heating vessel. Steam and methane flow through the externally heated tubes, which contain catalyst material.
Steam hydrocarbon reforming and autothermal reforming reactions can produce hydrogen, carbon monoxide, carbon dioxide, methane gases, and steam. The composition of the product gas, for instance hydrogen gas, fuel gas or various synthesis gas, depends upon the temperature, pressure and ratios of feed materials such as the steam/carbon ratio, which is defined as a ratio of moles of steam per gram-atom of carbon of the hydrocarbon.
In general, a steam reforming reaction and an autothermal reforming reaction involves a large scale endothermic and exothermic reaction, respectively. When steam reforming is carried out on an industrial scale, heat supply from outside raises technical problems. The overall reaction of industrially employed steam reforming of hydrocarbons, using, for instance, nickel catalysts, is considerably endothermic with the heat of reaction being 49.3 kcal/mol of methane at about 700° C. Although various attempts have been made towards improving the steam reforming reactor and heat recovery system itself, such improvements do not represent solutions for attaining enhanced heat recovery and simplified facilities. Therefore, focus has been directed towards production of more suitable catalysts.
It is preferred that the catalysts for SMR/ATR reactions are capable of high catalytic activity/selectivity and high mechanical strength/stability, in order to withstand normal process conditions. The ideal commercial catalyst would therefore satisfy the dual requirement of high activity/selectivity and long service life (e.g. good strength and stability), such properties become more difficult and specialized if the catalyst is used in a kinetic region and under fluidized conditions.
High catalytic activity is related to the specific surface area of the active metal of the catalyst and the accessibility of that surface area. Generally, the porosity, the pore size distribution, and the geometry of the active metal surface are such that access to the inner pore surface is achieved during the catalyzed reaction. It has been strongly believed that mechanical strength of the catalyst is indirectly proportional to the activity of the catalyst. Therefore, the higher the activity, the lower the mechanical strength. For instance, when the mechanical strength of the catalyst is important, impregnation of low surface area preformed refractory material, such as α-alumina or silica, with the active metal is commonly practiced. Such supported catalysts normally have a low active surface area and consequently, have a lower activity and a lower catalyst life.
The various catalysts produced by existing catalyst technologies, such as impregnation of a preformed carrier, in fact represent compromise between activity and strength. There is a need for a catalyst capable of achieving both a high activity and a good strength.
Other problems associated with existing reforming catalysts include carbon deposition onto the catalyst during the reforming reaction. Carbon deposition onto the catalyst not only lowers the activity of the catalyst but also, over time, even more detrimentally, causes the catalyst to lower its abrasion resistance, disintegrate and block gas conduits, shutting down the reforming process.
To prevent carbon deposition, the steam/carbon ratio is usually increased since low steam/carbon ratios increases the risk that carbon will be deposited on the catalyst, resulting in a loss of activity. An increase in the steam/carbon ratio suppresses the carbon deposition onto the catalyst, which leads to consumption of the feed materials, fuel, etc. Thus, the increase in the steam/carbon ratio would not be an economical way of controlling carbon deposition.
Several catalysts have been formulated to prevent carbon deposition during reforming reactions. U.S. Pat. No. 4,060,498 to Kawagoshi et al. is directed to a process for steam reforming hydrocarbons using a specific type of catalyst to prevent carbon deposition. The catalyst comprises at least 3% by weight, preferably 10 to 30% by weight, of nickel per weight of the catalyst; at least 2 mg-atoms of silver per 100 g of the catalyst; at least one rare-earth element in an atomic ratio of the rare-earth elements to silver of 10 or less, preferably 0.2 to 2.0; and a heat-resistant oxide carrier such as alumina. It is specifically taught that if there is less that 2 mg-atoms of silver per 100 g of the catalyst then the suppression of carbon deposition is not satisfied.
Silver metal is known to lose its stability at higher temperatures and is normally used to convert certain gases to their respective oxides. Consequently, a silver-containing catalyst produces a product stream that includes more carbon monoxide and carbon dioxide, which is more hazardous to the environment.
International Patent Application No. WO 99/47257 to Lallje et al. is directed to a steam reforming catalyst that includes from about 50% to about 75% nickel oxide, from about 5% to about 12% of an alkaline earth oxide, from about 10% to about 40% of a support material and from about 4% to about 20% of a rare-earth oxide promoter. Such a catalyst is not useful for ATR reactions.
Other problems with existing reforming catalysts include the production of these catalysts. It is taught throughout the art to avoid the production of a supported catalyst containing metal aluminates since such catalysts are difficult to reduce and have negligible activity. U.S. Pat. No. 4,962,280 to Tijburg and Geus is directed to a process for making a catalyst that involves, for example, suspending alumina in water and adjusting the pH to 5 by adding nitric acid, followed by the addition of lanthanum nitrate in an EDTA solution. The suspension produced was filtered and dried at high temperatures. Cobalt nitrate was added to a suspension of the dried alumina/lanthanum oxide. The pH of the solution of cobalt nitrate and alumina/lanthanum oxide was adjusted using nitric acid. The resulting solid was selectively heated to avoid the formation of cobalt aluminate.
Another problem with existing reforming catalysts involves sulfur poisoning. Trace amounts of sulfur found in feeds reacts with the active catalytic sites of the catalysts, ruining their activity. U.S. Pat. No. 4,215,998 to Futami is directed to a catalyst for production of methane-containing gases which is formed from a catalyst precursor composed of a mixed precipitate of hydroxides and/or carbonates of nickel, lanthanum and aluminum, which is obtained by stepwise addition of solutions of alkaline substances to a solution of an aluminum salt, a solution of a lanthanum salt, and a solution of a nickel salt. This stepwise addition of solutions of alkaline substances is accomplished by (1) first stage addition of a solution of an alkaline substance to a solution of an aluminum salt, (2) second stage addition of a solution of an alkaline substance to a solution of a lanthanum salt in the presence of the precipitate-containing solution formed by the first stage addition and (3) third stage addition of a solution of an alkaline substance to a solution of a nickel salt in the presence of the precipitate-containing solution formed by the second stage addition. The resulting precipitate is heated to 100-400° C. The catalyst, however, is not sulfur resistant. It is suggested that sulfur be removed from the hydrocarbon feed before being subjected to the steam-reforming reaction with this type of catalyst.
U.S. Pat. No. 4,539,310 to Leftin and Patil is directed to a steam reforming catalyst that is particularly useful for reforming feedstocks containing from a trace amount to about 5% by weight sulfur. The catalyst contains nickel oxide, rare-earth metal oxide, and zirconium oxide. Other refractory oxides may also be incorporated into the catalyst composition, such as alumina. It is taught, however, that care should be taken not to form spinel-type structures, such as nickel aluminate, since such formations reduce nickel content.
Oxidation is another concern with respect to the existing reforming catalysts. The chemistry of autothermal reforming is similar to that of steam methane reforming, but differs in that there is a concurrent partial oxidation step. Oxidation of the reforming catalyst will also attribute to its' reduced activity.
At present, two different types of catalysts are used for ATR and SMR reactions. There is a need for a reforming catalyst that may be used in both types of reactions.
There is also a need for a reforming catalyst that obviates or mitigates at least some of the disadvantages of the prior art catalysts and processes. For instance, there is a need for a reforming catalyst that is resistant to carbon deposition, sulfur poisoning and oxidation, even at low steam/carbon ratios. In addition, there is a need for a reforming catalyst that may be used at a wide range of temperatures and pressures and still maintain a high activity. There is also a need for a catalyst that has a high attrition resistance such that the catalyst would work in a fluidized bed system.